With much of the world's easily obtainable oil having already been produced, new techniques are being developed to extract less accessible hydrocarbons. These techniques often involve drilling a borehole in close proximity to one or more existing wells. Examples of directed drilling near an existing well include well intersection for blowout control, multiple wells drilled from an offshore platform, and closely spaced wells for geothermal energy recovery. Another such technique is steam-assisted gravity drainage (SAGD) that uses a pair of vertically-spaced, horizontal wells constructed along a substantially parallel path, often less than ten meters apart. Careful control of the spacing contributes to the effectiveness of the SAGD technique.
One way to construct a borehole in close proximity to an existing well is “active ranging” or “access-dependent ranging” in which an electromagnetic source is located in the existing well and monitored via sensors on the drill string in the well under construction. Another technique involves systems that locate both the source and the sensor(s) on the drill string—relying on backscatter transmission from the target well to determine the range between the drilling well and the target well. These latter systems are sometimes called “passive ranging” or “access-independent” systems by those of ordinary skill in the art. In either case, the ranging techniques are sometimes limited in the degree of accuracy that can be obtained.